Power amplifier and wireless communication device

ABSTRACT

A power amplifier includes an output signal generator constructed to generate, on the basis of an input AC signal, an output signal including, in cycles, a first pulse being width higher in voltage than a first reference voltage and a second pulse being width lower in voltage than the first reference voltage and a feedback circuit constructed to generate a first bias signal corresponding to the output signal and feed back the first bias signal to an input side of the output signal generator to equalize a width of the first pulse and a width of the second pulse in the cycles of the output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-007877, filed on Jan. 22, 2018; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments disclosed herein relate to a power amplifier and a wireless communication device.

BACKGROUND

A power amplifier including an inverter is used for wireless communication and the like. In the wireless communication, because a usable band is limited, it is demanded to prevent occurrence of even-order harmonic components included in an output signal of the power amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the configuration of a power amplifier according to a first embodiment;

FIG. 2 is a circuit diagram showing the configuration of a bias circuit;

FIGS. 3A to 3E are waveform charts for explaining the operation of the power amplifier according to the first embodiment;

FIGS. 4A to 4E are other waveform charts for explaining the operation of the power amplifier according to the first embodiment;

FIG. 5 is a graph for explaining the performance of the power amplifier according to the first embodiment;

FIG. 6 is a circuit diagram showing the configuration of a power amplifier according to a second embodiment;

FIG. 7 is a circuit diagram showing the configuration of a power amplifier according to a third embodiment; and

FIG. 8 is a diagram showing a connection state example during normal operation of the power amplifier according to the third embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention are explained below with reference to the drawings. Note that, in the drawings appended to this specification, for convenience of illustration and easiness of understanding, scales, aspect ratios, and the like are changed from actual ones and exaggerated.

First Embodiment

The configuration of a power amplifier is explained with reference to FIG. 1. FIG. 1 is circuit diagram showing the configuration of a power amplifier according to the first embodiment.

As shown in FIG. 1, the power amplifier includes a signal generator 1, an inverter 2, a feedback circuit 3, a bias supply circuit 4, a matching circuit 5, and an antenna 6. The feedback circuit 3 includes a detector 31, an adjuster 32, and an electric resistor 33. The detector 31 includes an electric resistor 31 a and a capacitor 31 b. The adjuster 32 includes a differential amplifier 32 a and a bias circuit 32 b. The bias supply circuit 4 includes a bias circuit 41 and an electric resistor 42.

The power amplifier shown in FIG. 1 further includes a first capacitor 11, a second capacitor 12, a first inverter 13, a second inverter 14, a third inverter 15, a fourth inverter 16, and a plurality of output signal generators 17 connected in parallel to one another.

The power amplifier according to this embodiment is used for wireless transmission conforming to, for example, the Bluetooth (registered trademark) standard. FIG. 1 shows components related to explanation of the wireless transmission in this embodiment. Illustration of known components unrelated to the explanation of this embodiment is omitted. Note that the matching circuit 5 and the antenna 6 may be components on the outside of the power amplifier according to this embodiment. The power amplifier shown in FIG. 1 can be implemented in a wireless communication device. When the power amplifier is implemented in the wireless communication device, the matching circuit 5 and the antenna 6 may be implemented in the same wireless communication device.

The signal generator 1 is a circuit that generates a signal and is, for example, a synthesizer or a digitally controlled oscillator (DCO). In FIG. 1, this signal is indicated by a sign V₁. The signal generator 1 generates a sine wave as the signal V₁. As the signal V₁ is propagated downstream, a waveform of the signal V₁ becomes closer to a rectangular wave because of the influence of inverters 2, 13, 14, 15, 16, and the like.

After passing through the inverter 2, in an input node K₁, the signal V₁ separates into a first signal supplied to the first capacitor 11 and a second signal supplied to the second capacitor 12. The first and second capacitors 11 and 12 respectively remove direct-current components (DC components) of the first and second signals.

After passing through the first and third inverters 13 and 15, the first signal passed through the first capacitor 11 is supplied to a gate terminal of a first transistor 17 a. After passing through the second and fourth inverters 14 and 16, the second signal passed through the second capacitor 12 is supplied to a gate terminal of a second transistor 17 b. Note that the number of inverters between the first capacitor 11 and the first transistor 17 a and between the second capacitor 12 and the second transistor 17 b is not limited to two and only has to be an even number.

The output signal generators 17 generate an output signal V₅ including, in cycles, a first pulse having a pulse width higher than a first reference voltage Vref by a first voltage and a second pulse having a pulse width lower than the first reference voltage Vref by a second voltage. The plurality of output signal generators 17 are connected in parallel to one another in the positions of frame lines surrounding the output signal generators 17. That is, a wire for inputting signals to the output signal generators 17 branches in nodes between the third inverter 15 and the output signal generators 17 and nodes between the fourth inverter 16 and the output signal generators 17. Wires for outputting signals from the output signal generators 17 merge in nodes between the output signal generators 17 and the matching circuit 5. Note that at least one output signal generator 17 only has to be provided. The number of the output signal generators 17 is adjusted according to characteristics of the antenna 6.

The feedback circuit 3 applies a first bias signal Vgp to the first signal such that a value of an average DC voltage Vdet of the output signal V₅ matches the first reference voltage Vref. In FIG. 1, the first signal immediately after the first bias signal Vgp is applied to a node K₂ between the first capacitor 11 and the first inverter 13 is indicated by a sign V_(2p). The first signal after passing through the first inverter 13 is indicated by a sign V₃p. The first signal after passing through the third inverter 15 is indicated by a sign V_(4p).

The bias supply circuit 4 applies a second bias signal Vgn to the second signal in a node K₃ between the second capacitor 12 and the second inverter 14. In FIG. 1, the second signal immediately after the second bias signal Vgn is applied is indicated by a sign V_(2N). The second signal after passing through the second inverter 14 is indicated by a sign V_(3N). The second signal after passing through the fourth inverter 16 is indicated by a sign V_(4N). The bias supply circuit 4 adjusts a signal level of the second bias signal Vgn to thereby control the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ of the output signal generators 17.

A detailed configuration of the output signal generator 17 is explained. The output signal generator 17 includes the first transistor 17 a and the second transistor 17 b having different conduction types from each other, cascode-connected between a first supply voltage node (K_(VDD)) and a second supply voltage node (K_(GND)). For example, the first and second transistors 17 a and 17 b are respectively a pMOS and an nMOS. The output signal generator 17 generates, on the basis of the signal V₁, an output signal including, in cycles, a first pulse having a pulse width higher than the first reference voltage Vref by a first voltage and a second puke having a pulse width lower than the first reference voltage Vref by a second voltage. The first reference voltage Vref is, for example, a half of a voltage VDD of a VDD wire. The first voltage and the second voltage have equivalent values. More specifically, the first transistor 17 a includes a first control terminal (a gate) to which the signal V₄ corresponding to the AC signal V₁ is input and a first output terminal (a drain) that outputs the first pulse at each cycle of the output signal V₅. The second transistor 17 b includes a second control terminal (a gate) to which the signal V_(4N) corresponding to the AC signal V₅ is input and a second output terminal (a drain) that outputs the second pulse at each cycle of the output signal V₅. The first output terminal (the drain) and the second output terminal (the drain) are connected to each other in a node K₄.

With such a circuit configuration, when a first signal V_(4P) is supplied, the first transistor 17 a outputs a first pulse current I_(1P). When the second signal V_(4N) is supplied, the second transistor 17 b outputs a second pulse current I_(1N). The first and second pulse currents I_(1p) and I_(1N) are respectively equivalent to drain currents of the first and second transistors 17 a and 17 b. As a result, the output signal V₅ including, in cycles, the first pulse and the second pulse is output from the output node K₄ between the first and second transistors 17 a and 17 b to the matching circuit 5. That is, the output signal V₅ is equivalent to a voltage in the output node K₄. The output signal V₅ is generated in the output signal generators 17 on the basis of the first pulse current I_(1P) and the second pulse current I_(1N) and output to the matching circuit 5.

Details of the feedback circuit 3 are explained. The feedback circuit 3 feeds back the first bias signal Vgp to a signal line connected to one of the first control terminal (the gate) of the first transistor 17 a and the second control terminal (the gate) of the second transistor 17 b. That is, the feedback circuit 3 generates the first bias signal Vgp corresponding to the output signal V₅ and feeds back the first bias signal Vgp to an input side of the output signal generator 17 such that the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are equalized.

The output signal V₅ is supplied to the detector 31 of the feedback circuit 3 as well. The detector 31 detects the average DC voltage Vdet of the output signal V₅. The detector 31 is, for example, a low-pass filter including the electric resistor 31 a and the capacitor 31 b and removes a high-frequency component of the output signal V₅. The output signal V₅, from which the high-frequency component is removed, is output to the adjuster 32 as the average DC voltage Vdet. Note that the detector 31 may have other configurations if the detector 31 is capable of removing the high-frequency component of the output signal V₅.

The adjuster 32 includes the differential amplifier 32 a and the bias circuit 32 b. The average DC voltage Vdet is input to a non-inverting input terminal (+) of the differential amplifier 32 a from the detector 31. The first reference voltage Vref is input to an inverting input terminal (−) of the differential amplifier 32 a from the bias circuit 41. The differential amplifier 32 a is, for example, an operational amplifier and generates the first bias signal Vgp such that a value of the average DC voltage Vdet and a value of the first reference voltage Vref match. When a value of the first bias signal Vgp is changed, a pulse width of the first signal V_(4P) is also changed. In other words, the adjuster 32 changes the pulse width of the first signal V_(4P) such that the average DC voltage Vdet and the first reference voltage Vref match. Note that a reference voltage generator according to this embodiment corresponds to the bias circuit 32 b.

FIG. 2 is a circuit diagram showing the configuration of the bias circuit 32 b. As shown in FIG. 2, the bias circuit 32 b includes a first variable resistor 32 c and a second variable resistor 32 d connected in series between the VDD wire and a GND wire. The bias circuit 32 b can control the value of the first reference voltage Vref by changing resistance values of the first variable resistor 32 c and the second variable resistor 32 d on the basis of an input code. Note that the bias circuit 32 b includes the first variable resistor 32 c and the second variable resistor 32 d. However, the bias circuit 32 b may have other configurations if the bias circuit 32 b is capable of controlling the value of the first reference voltage Vref. The bias circuit 41 has a configuration equivalent to the configuration of the bias circuit 32 b.

The first reference voltage Vref is set to an average DC voltage that should be obtained if the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are equal. The average DC voltage is, for example, a half of the voltage VDD of the VDD wire. Therefore, the first reference voltage Vref is set to VDD/2. In a value of the average DC voltage at the time when the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are equal, deviation of a could occur according to characteristics of the first and second transistors 17 a and 17 b. Therefore, the first reference voltage Vref may be set to VDD/2+α on the basis of, for example, the voltage of the VDD wire and the characteristics of the first and second transistors 17 a and 17 b. For example, the value of the first reference voltage Vref may be acquired in advance by a preliminary experiment.

The matching circuit 5 is provided for impedance matching between the output signal generator 17 and the antenna 6. The output signal V₅ of the output signal generator 17 is supplied to the antenna 6 passing through the matching circuit 5 and transmitted to the outside from the antenna 6.

In this way, the power amplifier according to this embodiment detects, with the detector 31, the average DC voltage Vdet of the output signal V₅ and changes, with the adjuster 32, a waveform of the output signal V₅ on the basis of the average DC voltage Vdet. Consequently, the power amplifier can adjust the waveform of the output signal V₅ such that the values of the average DC voltage Vdet and the first reference voltage Vref match. More specifically, the power amplifier according to this embodiment operates to match the value of the average DC voltage of the output signal V₅ with VDD/2, match the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅, and make the waveforms of the output signal V₅ in the cycles point symmetrical. Points of symmetry are set to points in time of half cycles in the cycles. Consequently, the waveform of the output signal V₅ becomes an odd function with respect to the points of symmetry. Even-order harmonics components are reduced to substantially zero.

The operation of the power amplifier according to this embodiment is explained in detail below.

FIGS. 3A to 3E are waveform charts for explaining the operation of the power amplifier according to the first embodiment. It is assumed that, when the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ match, the output signal V₅ is referred to as being symmetrical. In this case, the first pulse and the second pulse are point symmetrical with respect to the points in time of the half cycles of the cycles, that is, the points of symmetry. The first reference voltage Vref is set to VDD/2.

FIG. 3A shows an example of the output signal V₅ before the feedback circuit 3 acts thereon and the output signal V₅ after the feedback circuit 3 acts thereon. The width of the first pulse is indicated by P₁ and the width of the second pulse is indicated by P₂. Values of the average DC voltage Vdet and the first reference voltage Vref of the output signal V₅ before the action are different. In the output signal V₅ before the action, the pulse width P₁ of the first pulse and the pulse width P₂ of the second pulse do not match (the left in FIG. 3A). That is, the symmetry of the output signal V₅ is broken. On the other hand, the average DC voltage Vdet and the first reference voltage Vref after the action match. The pulse width P₁ of the first pulse and the pulse width P₂ of the second pulse match (the right in FIG. 3A). The feedback circuit 3 matches the average DC voltage Vdet and the first reference voltage Vref in this way, whereby the waveform of the output signal V₅ becomes point symmetrical,

FIG. 3B shows an example of the first signal V_(4P) before the feedback circuit 3 acts thereon and the first signal V_(4P) after the feedback circuit 3 acts thereon. In the first signal V_(4P) before the action, a high period and a low period have substantially equal lengths (the left in FIG. 3B). On the other hand, in the first signal V_(4P) after the action, the high period changes to be longer than the low period (the right in FIG. 3B).

FIG. 3C shows an example of the second signal V_(4N) before the feedback circuit 3 acts thereon (the left in FIG. 3C) and the second signal V_(4N) after the feedback circuit 3 acts thereon (the right in FIG. 3C).

FIG. 3D shows an example of the first pulse current I_(1P) before the feedback circuit 3 acts thereon and the first pulse current I_(1P) after the feedback circuit 3 acts thereon. The first signal V_(4P) changes as shown in FIG. 3B, whereby a pulse width of the first pulse current I_(1P) after the action decreases.

FIG. 3E shows an example of the second pulse current I_(1N) before the feedback circuit 3 acts thereon and the second pulse current I_(1P) after the feedback circuit 3 acts thereon. A pulse width of the second pulse current I_(1N) after the action is also maintained without changing because the second signal V_(4N) is maintained without changing as shown in FIG. 3C.

In this way, the first bias signal V_(gp) is changed such that the pulse widths P₁ and P₂ of the first pulse and the second pulse of the output signal V₅ match. Consequently, the widths P₁ and P₂ of the first pulse and the second pulse match. As a result, even-order harmonic components included in the output signal V₅ are reduced and even-order harmonic components included in a signal output to the antenna 6 are reduced.

The bias supply circuit 4 adjusts the second bias signal Vgn applied to the second signal, whereby it is possible to change the waveform of the output signal V₅ while maintaining a state in which the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are matched. Consequently, it is possible to change an amount of a through-current flowing in the first and second transistors 17 a and 17 b. It is possible to adjust energy efficiency of wireless communication in this embodiment.

FIGS. 4A to 4E are other waveform charts for explaining the operation of the power amplifier according to the first embodiment. FIGS. 4A to 4E show a relation between application of the first bias signal Vgp by the feedback circuit 3 and a change in the first pulse current I_(1P).

FIG. 4A shows an example of the signal V₁ generated from the signal generator 1. The signal V₁ is a sine wave that fluctuates between a voltage 0 and the voltage VDD. A first voltage and a first pulse current generated from the signal V₁ are explained below.

FIG. 4B shows a first signal V_(2P) immediately after the first bias signal Vgp is applied. A vibrating direction of the first signal V_(2P) is reversed from a vibrating direction of the signal V₁ by the action of the inverter 2. An average of the first signal V_(2P) is higher than a threshold V_(TH) of the first inverter 13 by V_(B) because of the influence of the first bias signal Vgp. Further, FIG. 4B shows a period T₁ in which a value of the first signal V_(2P) is lower than the threshold V_(TH) and a period T₂ in which the value of the first signal V_(2P) is higher than the threshold V_(TH).

FIG. 4C shows a first signal V_(3P) after passing through the first inverter 13. Because of the influence of the threshold V_(TH), a waveform of the first signal V_(3P) changes to a rectangular wave in which the period T₁ is shorter than the period T₂. Note that, accurately, the first signal gradually changes from the sine wave to the rectangular wave as the first signal is propagated downstream. However, for convenience of drawing, the first signal V_(2P) is indicated by the sine wave and the first signal V₃ is indicated by the rectangular wave.

FIG. 4D shows a first signal V_(4P) after passing through the third inverter 15. Because of the action of the third inverter 15, a waveform of the first signal V_(4P) changes to a rectangular wave in which the period T₁ is shorter than the period T₂.

FIG. 4E shows the first pulse current I_(1P) output from the first transistor 17 a. A pulse width of the first pulse current I_(1P) changes to T₁ because of the influence of the period T₁ of the first signal V_(4P).

In this way, the period T₁ of the first signal V_(4P) changes according to the first bias signal Vgp. Consequently, the pulse width of the first pulse current I_(1P) changes. As a result, as shown on the right of FIG. 3A, the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ can be matched. Consequently, the waveforms of the output signal V₅ (FIG. 1) in the cycles become point symmetrical.

FIG. 5 is a graph for explaining the performance of the power amplifier according to the first embodiment. A power supply voltage VDD, temperature, and a frequency of an input signal V1 were respectively changed and a computer experiment for calculating harmonic components of the output signal V₅ was performed. The upper side of FIG. 5 shows a result obtained when the feedback circuit 3 is not used. The lower side of FIG. 5 shows a result obtained when the feedback circuit 3 is used. The vertical axis of FIG. 5 indicates power of harmonics of the output signal V₅. The horizontal axis of FIG. 5 indicates a frequency of the output signal V₅. Respective lines indicate results obtained by changing the power supply voltage VDD, the temperature, and the frequency of the input signal V1.

As shown in FIG. 5, when the feedback circuit 3 is not used, a value of the harmonic components of the output signal V₅ shows fluctuation with respect to fluctuation in the power supply voltage VDD, the temperature, and the frequency of the input signal V1. On the other hand, when the feedback circuit 3 is used, even if the power supply voltage VDD, the temperature, and the frequency of the input signal V1 fluctuate, the fluctuation in the value of the harmonic components of the output signal V₅ is shown smaller than when the feedback circuit 3 is not used. As it is seen from this, by using the feedback circuit 3, even if the power supply voltage VDD, the temperature, and the frequency of the input signal V1 fluctuate, it is possible to robustly reduce the harmonic components of the output signal V₅. In other words, by using the feedback circuit 3, even if the power supply voltage VDD, the temperature, and the frequency of the input signal V1 fluctuate, it is possible to prevent the symmetry of the waveform of the output signal V₅ from being broken.

As explained above, according to this embodiment, when the first reference voltage Vref is set to the value of the average DC voltage obtained when the waveform of the output signal V₅ becomes symmetrical, the feedback circuit 3 changes the first bias signal Vgp such that the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are equalized. Consequently, the symmetry of the waveform of the output signal V₅ is improved. The even-order harmonic components are prevented from being included in the output signal V₅.

Second Embodiment

FIG. 6 is a circuit diagram showing the configuration of a power amplifier according to a second embodiment. The power amplifier according to the second embodiment is different from the power amplifier according to the first embodiment in that the feedback circuit 3 applies the second bias signal Vgn to the second signal in the node K₃ between the second capacitor 12 and the second inverter 14. The power amplifier according to the second embodiment is different from the power amplifier according to the first embodiment in that the bias supply circuit 4 applies the first bias signal Vgp to the first signal in the node K₂ between the first capacitor 11 and the first inverter 13. The other components are equivalent to the components of the power amplifier according to the first embodiment. Therefore, the equivalent components are denoted by the same reference numerals and signs and explanation of the components is omitted.

According to this embodiment, when the first reference voltage Vref is set to the value of the average DC voltage obtained when the waveform of the output signal V₅ becomes symmetrical, the feedback circuit 3 changes the second bias signal Vgn such that the pulse widths of the first pulse and the second pulse in the cycles of the output signal V₅ are equalized. Consequently, the symmetry of the waveform of the output signal V₅ is improved. The even-order harmonic components are prevented from being included in the output signal V₅.

Third Embodiment

The power amplifier according to the third embodiment is different from the power amplifier according to the second embodiment in that the power amplifier according to the third embodiment further includes a pre-driver 7 used during startup of the power amplifier. When the feedback circuit 3 is driven during the startup of the power amplifier, it is likely that the feedback circuit 3 performs an unstable operation and an unnecessary wave component occurs in an output signal of the power amplifier. Therefore, the power amplifier according to this embodiment electrically separates the feedback circuit 3 from the power amplifier during the startup of the power amplifier and causes the pre-driver 7 to pre-drive the differential amplifier 32 a.

FIG. 7 is a circuit diagram showing the configuration of the power amplifier according to the third embodiment. In FIG. 7, a connection state example during the pre-drive of the differential amplifier 32 a is shown. FIG. 8 is a circuit diagram showing a connection state example during normal operation of the power amplifier according to the third embodiment.

As shown in FIG. 7, the power amplifier according to the third embodiment includes the pre-driver 7, a bias circuit 8, and changeover switches 18 and 19. Note that the power amplifier according to the third embodiment may include the pre-driver 7, the bias circuit 8, and the changeover switches 18 and 19 in addition to the components shown in FIG. 1. In this case, control equivalent to the pre-drive according to this embodiment can be performed on the differential amplifier 32 a.

In the pre-drive of the differential amplifier 32 a, the pre-driver 7 controls a voltage input to the non-inverting input terminal of the differential amplifier 32 a to be the first reference voltage Vref and controls an output voltage of the differential amplifier 32 a to be an initial voltage Vbn0.

The bias circuit 8 has a configuration equivalent to the configuration of the bias circuit 32 b. The bias circuit 8 supplies the initial voltage Vbn0 to the node K₃ during the pre-drive of the differential amplifier 32 a. Consequently, the initial voltage Vbn0 is supplied to the gate of the second transistor 17 b. On the other hand, the bias circuit 41 supplies an initial voltage Vbp0 to the node K₂ during the pre-drive of the differential amplifier 32 a. The initial voltages Vbn0 and Vbp0 are values set by a preliminary experiment in advance.

As shown in FIG. 8, in the connection state during the normal operation of the power amplifier, when the initial voltage Vbp0 is applied to the node K₂, the output voltage of the differential amplifier 32 a reaches near Vbn0. In other words, an output voltage normally output by the differential amplifier 32 a when the initial voltage Vbp0 is supplied to the node K₂ is estimated by the predetermined experiment and set as the initial voltage Vbn0.

The changeover switches 18 and 19 electrically separate the adjuster 32 from the power amplifier during the pre-drive and electrically connects the adjuster 32 to the power amplifier during the normal operation. That is, the changeover switches 18 and 19 switch connection of the pre-driver 7, the bias circuit 8, and the adjuster 32. More specifically, as shown in FIG. 7, the changeover switches 18 and 19 connect the adjuster 32 to the pre-driver 7 and connect the bias circuit 8 to one end of the electric resistor 33 when the differential amplifier 32 a is pre-driven. On the other hand, as shown in FIG. 8, when differential amplifier 32 a is normally driven, the changeover switches 18 and 19 connect the adjuster 32 to one end of the electric resistor 33 and one end of the detector 31. Note that a switcher according to this embodiment corresponds to the changeover switches 18 and 19.

A detailed configuration of the pre-driver 7 is explained. The pre-driver 7 includes a third transistor 70 a, a fourth transistor 70 b, a fifth transistor 70 c, a sixth transistor 70 d, and a differential amplifier 70 e.

The third transistor 70 a and the fourth transistor 70 b are connected in series between a power supply wire (a VDD wire) and a ground wire (a GND wire). Similarly, the fifth transistor 70 c and the sixth transistor 70 d are connected in series between the power supply wire (the VDD wire) and the ground wire (the GND wire). The third transistor 70 a and the fifth transistor 70 c are pMOSs having equivalent characteristics. The fourth transistor 70 b and the sixth transistor 70 d are nMOSs having equivalent characteristics.

The differential amplifier 70 e is, for example, an operational amplifier. A connection node K₇ of the third transistor 70 a and the fourth transistor 70 b is connected to one end of the changeover switch 18. A gate of the fourth transistor 70 b is connected to one end of the changeover switch 19. A connection node K₅ of the fifth transistor 70 c and the sixth transistor 70 d is connected to a non-inverting input terminal (+) of the differential amplifier 70 e. The first reference voltage Vref is input to an inverting input terminal (−) of the differential amplifier 70 e. An output terminal of the differential amplifier 70 e is connected to gates of the third transistor 70 a and the fifth transistor 70 c.

The operation of the pre-driver 7 is explained. During driving, the node K₇ of the third transistor 70 a and the fourth transistor 70 b is connected to the non-inverting input terminal (+) of the differential amplifier 32 a by the changeover switch 18. The gate of the fourth transistor 70 b is connected to an output terminal of the differential amplifier 32 a by the changeover switch 19. The first reference voltage Vref is supplied to the inverting input terminal (−) of the differential amplifier 32 a. The initial voltage Vbn0 is supplied to the gate of the sixth transistor 70 d. The node K₃ is connected to the bias circuit 8 via the resistor 33 by the changeover switch 19. The initial voltage Vbn0 is supplied to the node K₃.

On the other hand, at normal time, one end of the detector 31 is connected to the non-inverting input terminal (+) of the differential amplifier 32 a by the changeover switch 18. The node K₃ is connected to the differential amplifier 32 a via the resistor 33 by the changeover switch 19.

As it is seen from the above, during the driving of the pre-driver 7, a voltage in the connection node K₆ changes to the first reference voltage Vref according to feedback action of the differential amplifier 70 e. On the other hand, an output voltage Vp7 of the differential amplifier 70 e is supplied to the gates of the third transistor 70 a and the fifth transistor 70 c. Therefore, when the voltage Vbn0 is supplied to the gate of the sixth transistor 70 d, a voltage in the connection node K₇ changes to the first reference voltage Vref as in the connection node K₆. When the pre-driver 7 in this state is connected to the adjuster 32, a voltage input to the non-inverting input terminal (+) of the differential amplifier 32 a changes to the first reference voltage Vref. The output voltage of the differential amplifier 32 a changes to the initial voltage Vbn0. The pre-driver 7 switches the changeover switches 18 and 19 and shifts to the operation at the normal time. That is, the pre-driver 7 sets a bias signal generated by the differential amplifier 32 a to a predetermined value (the initial voltage Vbn0) and thereafter feeds back the bias signal to the input side of the output signal generator 17. In this way, during the driving, in a state in which the bias signal generated by the differential amplifier 32 a is not fed back to the output signal generator 17, the pre-driver 7 brings the differential amplifier 32 a into a state close to the normal state.

As explained above, according to this embodiment, the feedback circuit 3 is electrically separated from the power amplifier during the startup of the power amplifier. When an operation state of the differential amplifier 32 a is brought into a state equivalent to an operation state at the normal time by the pre-driver 7, the feedback circuit 3 is connected to the power amplifier. Consequently, the power amplifier can be operated in a stable state from the startup time. An unnecessary wave component is prevented from occurring in an output signal of the power amplifier.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

1. A power amplifier comprising: an output signal generator constructed to generate, on the basis of an input AC signal, an output signal including, in cycles, a first pulse being width higher in voltage than a first reference voltage and a second pulse being width lower in voltage than the first reference voltage; and a feedback circuit constructed to generate a first bias signal corresponding to the output signal and feed back the first bias signal to an input side of the output signal generator to equalize a width of the first pulse and a width of the second pulse in the cycles of the output signal.
 2. The power amplifier according to claim 1, wherein the feedback circuit includes: a detector constructed to detect an average DC voltage of the output signal; and a differential amplifier constructed to generate the first bias signal such that the average DC voltage matches a first reference voltage.
 3. The power amplifier according to claim 2, further comprising: a switcher constructed to switch whether to feed back the first bias signal to the input side of the output signal generator; and a pre-driver constructed to stabilize an output signal of the differential amplifier in a state in which the first bias signal is not feed back to the input side of the output signal generator by the switcher, wherein the switcher causes the pre-driver to set the output signal of the differential amplifier to a predetermined value and thereafter feeds back the first bias signal.
 4. The power amplifier according to claim 1, wherein the output signal generator includes a first transistor and a second transistor having different conduction types from each other, the first transistor and the second transistor being cascode-connected between a first supply voltage node and a second supply voltage node, the first transistor includes a first control terminal to which a signal corresponding to the AC signal is input and a first output terminal constructed to output the first pulse at each cycle of the output signal, the second transistor includes a second control terminal to which a signal corresponding to the AC signal is input and a second output terminal constructed to output the second pulse at each cycle of the output signal, the first output terminal and the second output terminal are connected to each other, and the feedback circuit feeds back the first bias signal to a signal line connected to one of the first control terminal and the second control terminal.
 5. The power amplifier according to claim 4, further comprising a bias supply circuit constructed to supply a second bias signal to a signal line connected to the other of the first control terminal and the second control terminal.
 6. The power amplifier according to claim 1, further comprising a reference voltage generator constructed to adjust a voltage level of the first reference voltage such that the width of the first pulse and the width of the second pulse match.
 7. The power amplifier according to claim 1, wherein the output signal generator generates the output signal having a point-symmetrical waveform having time points of half cycles in the cycles as points of symmetry.
 8. The power amplifier according to claim 6, wherein the bias supply circuit adjusts a signal level of the second bias signal to thereby control the widths of the first pulse and the second pulse.
 9. A wireless communication device comprising a power amplifier, the power amplifier including: an output signal generator constructed to generate, on the basis of an input AC signal, an output signal including, in cycles, a first pulse being width higher in voltage than a first reference voltage and a second pulse being width lower in voltage than the first reference voltage; and a feedback circuit constructed to generate a first bias signal corresponding to the output signal and feed back the first bias signal to an input side of the output signal generator to equalize a width of the first pulse and a width of the second pulse in the cycles of the output signal.
 10. The wireless communication device according to claim 9, wherein the feedback circuit includes: a detector constructed to detect an average DC voltage of the output signal; and a differential amplifier constructed to generate the first bias signal such that the average DC voltage matches a first reference voltage.
 11. The wireless communication device according to claim 10, wherein the power amplifier further includes: a switcher constructed to switch whether to feed back the first bias signal to the input side of the output signal generator; and a pre-driver constructed to stabilize an output signal of the differential amplifier in a state in which the first bias signal is not feed back to the input side of the output signal generator by the switcher, and the switcher converges the output signal of the differential amplifier a predetermined value to the pre-driver and thereafter feeds back the first bias signal.
 12. The wireless communication device according to claim 9, wherein the output signal generator includes a first transistor and a second transistor having different conduction types from each other, the first transistor and the second transistor being cascode-connected between a first supply voltage node and a second supply voltage node, the first transistor includes a first control terminal to which a signal corresponding to the AC signal is input and a first output terminal constructed to output the first pulse at each cycle of the output signal, the second transistor includes a second control terminal to which a signal corresponding to the AC signal is input and a second output terminal constructed to output the second pulse at each cycle of the output signal, the first output terminal and the second output terminal are connected to each other, and the feedback circuit feeds back the first bias signal to a signal line connected to one of the first control terminal and the second control terminal.
 13. The wireless communication device according to claim 12, wherein the power amplifier further includes a bias supply circuit constructed to supply a second bias signal to a signal line connected to the other of the first control terminal and the second control terminal.
 14. The wireless communication device according to claim 11, wherein the power amplifier further includes a reference voltage generator constructed to adjust a voltage level of the first reference voltage such that the width of the first pulse and the width of the second pulse match.
 15. The wireless communication device according to claim 9, wherein the output signal generator generates the output signal having a point-symmetrical waveform having time points of half cycles in the cycles as points of symmetry.
 16. The wireless communication device according to claim 14, wherein the bias supply circuit adjusts a signal level of the second bias signal to thereby control the widths of the first pulse and the second pulse. 